The use of multiple motors connected to a single inverter drive is known in topside applications. Unlike with a single motor connected to a variable frequency drive (VFD), with multiple motors driven by the same VFD each motor may have its own overload and short circuit protection. When controlling a single motor, a VFD with adequate features can both provide short circuit and overload protection and will be able to sense an over current situation and take proper action to protect the motor.
With multiple smaller motors, connected to a single inverter drive output, the motor protection may be provided by individual relays. Larger motors might be powered by individual inverters, sometimes connected to a common rectifier and DC bus. However, none of these solutions are considered as practical in subsea applications.
Multiple subsea motors, operating from a topside single variable speed drive, have in the past been supplied via individual three phase cores in the umbilical cable system. In such cases the individual motor protection has been located topside, downstream from the inverter. However, this solution increases the number of cores in the umbilical system that in many cases is highly costly. Furthermore, if the transmission distance is long, both topside and subsea transformers may be highly beneficial or necessary. In such cases using separate three-phase umbilical cores, each subsea motor would need a separate transformer in order to be able to differentiate the pump motor currents and loading.